As electric circuit components become increasingly miniaturized and the applicable technologies of integrated circuit devices become increasingly varied, it becomes increasingly necessary to stably mount a semiconductor package on a printed circuit board. To mount a semiconductor package on a printed circuit board, solder joints are often formed on the surface of the printed circuit board where the semiconductor package will be mounted. The semiconductor package is then placed on the printed circuit board at the location of the solder joints. Subsequently, a reflow process is performed at a high temperature to make the semiconductor package and the printed circuit board stick to each other via the solder joints.
FIG. 1 is a cross-sectional diagram illustrating a conventional semiconductor package, printed circuit board, and solder joints before a reflow process. FIG. 2 is a cross-sectional diagram showing the same structure after the reflow process. As shown in FIG. 1, a thermal-mechanical stress is not applied to solder joints 30 positioned between a printed circuit board 10 and a semiconductor package 20 before the reflow process. Thus, the solder joints 30 have a thickness d11 and are maintained a distance d21 apart from adjacent solder joints 30.
As shown in FIG. 2, when the reflow process is performed at a predetermined temperature (e.g., at about 200-300° C.) for a predetermined time, a thermal-mechanical stress is applied and the solder joints 35 become pressed down to the right and the left. As a result, the solder joints 35 have a second thickness d12, which is thinner than the thickness d11 of the solder joints 30 before the reflow process. After the re-flow process, adjacent solder joints 35 are spaced apart by a distance d22, which is narrower than the space d21 of the solder joints 30 before the reflow process.
As the thickness of solder joints 35 becomes thinner, a decrease in reliability (such as decreasing fatigue life) can result. For example, where the printed circuit board 10 and the semiconductor package 20 are made of different materials (and thus expand at different rates due to their different thermal expansion coefficients), the solder joints 35 should be modified appropriately to connect the printed circuit board 10 and the semiconductor package 20. If the solder joints 35 are thin, however, the range of such modification is limited and the solder joints may even be severed, thus reducing reliability of a device. In another example, the solder joints 35 may have different thicknesses and the attached semiconductor package 20 can become tilted. Thus, the fatigue life of a particular solder joint applied with relatively more stress will be shortened considerably. Consequently, the reliability of a device is damaged.